1. Field of the Invention
The present invention is directed to a process and an apparatus for producing a corrugation-free foam. Specifically, the foaming process includes the injection and mixing of a predominantly carbon dioxide physical blowing agent into a foamable extrudate and extruding the mixture through an annular die and contacting a restrictive surface within a critical time after the foam exits.
2. Background of the Invention
Low density polymeric foam, such as polystyrene foam, is conventionally made by combining a physical blowing agent with a molten polymeric mixture under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere. This type of foam is commonly used to manufacture plates, bowls, cups and like items.
From about the 1950's to the present, physical blowing agents of choice have included halocarbons, hydrocarbons, specific atmospheric gases, or combinations thereof. Examples of the halocarbons include commercially available halocarbon compositions such as dichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11) and mixtures thereof. Examples of the hydrocarbon blowing agents are the C2–C6 alkanes such as ethane, propane, butane, isobutane, pentane, isopentane, and hexane. Examples of the specific atmospheric gases are carbon dioxide and argon.
During the 1980's, the worldwide scientific community presented evidence linking halocarbons containing halogens other than fluorine, such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) with atmospheric ozone depletion. Consequently, governments sought to regulate CFCs and HCFCs. As a result of such regulations, manufacturers of extruded polymeric foam products were forced to switch to alternatives which have had adverse effects that resulted in increased processing costs, reduced product quality, and increased safety issues, or combinations thereof.
For example, hydrocarbon blowing agents, particularly the short-chained alkanes produce foam with satisfactory physical properties. However, depending upon the location of the factory and the amount of the blowing agent used, a manufacturer may be required to capture and destroy emissions of the hydrocarbon blowing agents through a processing step like incineration. Atmospheric emission of short-chained hydrocarbons, which are classified as photoreactive volatile organic compounds (VOCs), when combined with certain other gases and subjected to sunlight, may result in “smog”. Moreover, the flammability of the hydrocarbons requires elaborate control systems and costly ventilation systems to prevent the exposure of highly flammable blowing agent-and-air mixtures to ignition sources. Similar to hydrocarbon blowing agents, certain hydrofluorocarbon blowing agents, such as 1,1-difluoroethane (HFC-152a), produce foam with satisfactory physical properties, but have the adverse effect of flammability. In addition, the nearly ten-fold higher unit pricing of these hydrofluorocarbon blowing agents in relation to most of the hydrocarbon blowing agents adversely increases foam product costs.
The disadvantages of the prior blowing agents have led to the use of carbon dioxide as a blowing agent. Carbon dioxide does not have the adverse environmental and flammability characteristics associated with CFCs and HCFCs. Carbon dioxide has a molecular weight that is lower than most of the commercially used hydrocarbons and the hydrofluorocarbons and thereby requires lower usage rates. Carbon dioxide also has lower unit pricing than the commercially used hydrocarbons and hydrofluorocarbons. However, the foams made with higher levels of carbon dioxide have not been comparable to foams made with hydrocarbon or with hydrofluorocarbon blowing agents. The foams made with blowing agents that are primarily carbon dioxide have generally had both increased product cost and decreased product quality. The increased cost is attributable to a combination of reduced extrusion rates and limited post-expansion in secondary operations, which results in increased product weight. The reduced product quality is attributable to both diminished aesthetics and increased variability in the mechanical properties.
The diminished aesthetics of foam produced with carbon dioxide is generally related to larger cell size, often greater than 0.4 mm, which give such foams a poor, grainy texture, and to the presence of multiple visible parallel regions of light and dark in the foam substrate. These adjacent parallel regions are not only deleterious to the visible aesthetics of the foams, but also create significant localized differences to mechanical properties.
The physical property that both diminishes aesthetics and increases the variability of the mechanical properties of foams made with carbon dioxide is related to the low solubility of the carbon dioxide gas in the polymer at ambient atmospheric conditions. The low solubility results in a very high volumetric expansion rate of the foamable composition at the die. As a consequence of this high volumetric expansion rate of the foam at the die, the use of a physical blowing agent comprised primarily of carbon dioxide in the production of fine-celled foams having a foam density below about 100 kg/m3 or a cell size below about 0.40 mm causes corrugation. The severity of the corrugations tends to increase as either the density or the cell size is decreased. The corrugations are manifest as periodic bands which are oriented in the machine direction within the extruded foam sheet and which differ in cell size, cell shape, and often foam thickness from the majority of the foam. The corrugations not only detract from the aesthetics but also reduce the overall mechanical properties of parts made from the foam.
In most food service and beverage applications, it is preferred that the average cell size be about 0.20–0.30 mm, which provides the foam with an aesthetically pleasing, relatively smooth surface texture while maintaining the requisite mechanical properties strength. Smaller cell sizes tend to undesirably sacrifice a smoother finish for strength. Larger cell sizes tend to have an undesirable appearance.
When used as the sole blowing agent, carbon dioxide's very high volumetric expansion rate typically produces unacceptable levels of corrugation. Therefore, previous attempts to use carbon dioxide as a blowing agent to produce a commercially acceptable foam product focused on blending the carbon dioxide with another blowing agent. The blended blowing agents typically included carbon dioxide as a minor constituent and either a hydrocarbon or hydrofluorocarbon blowing agent as the predominant constituent. A common blended blowing agent would include carbon dioxide in combination with pentane. Typically, the carbon dioxide in the blended blowing agent was limited to 30 mole percent of the blended blowing agent, which reduced, but did not eliminate, the use of a hydrocarbon or hydroflurocarbon-blowing agent. Thus, the blended blowing agent still has the disadvantages of the hydrocarbon and hydroflurocarbon blowing agents.
Attempts were also made to produce a commercially suitable polystyrene foam with substantially 100 percent carbon dioxide as the blowing agent. Examples of such processes and foams are disclosed in U.S. Pat. Nos. 5,266,605, 5,340,844, and 5,250,577. Most of these foams had an average cell size of 0.36 mm and still contained visible corrugation. Although these foams would be suitable for some applications, they did not produce corrugation-free foams with cell sizes in the preferred range.
Referring to FIGS. 1 and 2, Applicants previously produced a corrugation-free polystyrene foam with 100 percent carbon dioxide as the blowing agent from a tandem extruder, which is commonly known in the industry, in combination with a choke ring annularly positioned around an annular die extrusion opening.
The two-stage extruder apparatus comprises a hopper A feeding material into a first extruder B where polystyrene resin material is heated and melted in heating zone C, mixed with a blowing agent delivered by an injector D, further mixed by a mixing zone E, and cooled in a second stage extruder in a cooling zone F before delivery to a die G. The choke ring H contacts the exiting extrudate before a sizing mandrel I sizes the sheet.
The previously-used choke ring 10 and die 12 are shown in more detail in FIG. 2. The choke ring 10 has a smooth temperature-regulated inner surface 11 that is positioned to be concentric with the die 12 so that extrudate 13 contacts inner surface 11 before reaching the sizing mandrel 15.
The previously-used die 12 comprises a first generally converging portion 16 that terminates at an annular die opening 18. A second converging portion 20 extends from the annular die opening 18 and terminates at a cylindrical portion 22.
The choke ring surface 11 and the annular die opening 18 are located a radius of rc and rd, respectively, from the longitudinal axis 24. The choke ring gap is the difference between the radii (rc−rd).
Two choke rings having different diameters were tried. The first choke ring had a diameter such that the gap was 0.2375 inches or 6.03 millimeters. The second choke ring had a gap of 0.18 inches or 4.57 millimeters, resulting in a contact time of approximately 0.37 ms for the given operational parameters. Although both of these choke rings produced corrugation-free foam, the cell size remained above 0.40 mm. Therefore, there is still a need for a polystyrene foam and method of making a polystyrene foam that is corrugation-free with a cell size in the preferred range and using a carbon dioxide blowing agent.